1. Field of the Invention
The invention pertains to a process and an apparatus for removing nitrates from a water stream as per the preamble to patent claims 1 and 6.
2. Discussion of Background
In the preamble, the invention relates to the prior art as known from the German Journal Chemie Ingenieur Technik 58 (1986) No. 12, pp. 938-945. In this, advantages and disadvantages of reverse osmosis and electrodialysis are compared. In electrodialysis, several non-selective anion exchange membranes and non-selective cation exchange membranes are arranged in series (module arrangement) between an anode made from platinum-plated steel and a cathode made from V2A steel. It is thereby theoretically possible to remove nitrate/chloride selectively, but for many cases such removal is inadequate in practice, since sulfate ions remain substantially in the diluate. In industry, the plant is operated at about 80% of the limiting current density, which is apparent from a sharp decrease in the pH. Any deposits on the membranes can be counteracted by periodically reversing the poles about every 4 hours and simultaneously switching the diluate and retentate chambers.
It was not stated that membranes having a selectivity for nitrate over sulfate are particularly suitable and that the selectivity increases with decreasing current density.
In the treatment of process water for the supply of drinking water, the level of toxic salts must be reduced to the extent that the residual level does not exceed the maximum values required by the authorities. Recently, the increasing nitrate content of the water (groundwater, spring water, river water etc.) to be used for providing process water and drinking water has caused difficulties.
The invention relates to the further development and improvement of the electrodialysis process for removing toxic salts in water, with the removal of nitrates at the highest possible water yield having priority.
In the more narrow sense, the invention relates to a process and an apparatus for removing nitrates from a water stream during the treatment of process water, based on an electrodialysis process which produces, on the one hand, a concentrate which has high concentrations of nitrate ions and is branched off from the water stream, and, on the other hand, leaves a diluate which contains low levels of nitrate ions and is to be used as process water.
The importance of reducing the nitrate content of ground water and drinking water is constantly increasing as a result of health and environmental legislation. Currently, the following processes are indicated in this respect for water treatment:
biological processes, PA1 ion-exchanger processes, and PA1 membrane processes. PA1 reverse osmosis and PA1 electrodialysis. PA1 H. Sontheimer and U. Rohmann, "Grundwasserbelastung mit Nitrat--Ursachen, Bedeutung, Losungswege" [Pollution of Groundwater with Nitrate--Causes, Importance and Remedies], gwf-wasser/abwasser, 125 (1984) 599-608, PA1 R. Rautenbach, W. Kopp, G. van Opbergen, T. Peters and R. Hellekes, "Elektrodialyse zur Nitratentfernung aus Grundwassern" [Electrodialysis for Removing Nitrates from Groundwaters], gwfwasser/abwasser, 126 (1985) 349-355, PA1 R. Rautenbach, W. Kopp, R. Hellekes, R. Peters and G. van Opbergen, "Separation of Nitrate from Well Water by Membrane Processes (Reverse Osmosis/Electrodialysis Reversal)", Aqua, 5 (1986) 279-282, PA1 M. Perr and O. Kedem, "La purification de l'eau par electrodialyse du nitrate" [The Purification of Water for Nitrates by Electrodialysis], Eau Ind., 55 (1981) 47-52, PA1 A. Eyal and 0. Kedem, "Nitrate-selective Anion-Exchange Membranes", J. Membrane Sci., 38 (1988) 101-111, PA1 D. A. Cowan and J. H. Brown, "Effect of Turbulence on Limiting Current in Electrodialysis Cells", Ind. Eng. Chem., 51 (1959) 1445-1448, PA1 DE-A-3,041,209, PA1 DE-A-2,855,775 PA1 U.S. Pat. No. 3,510,417, PA1 U.S. Pat. No. 3,510,418.
The membrane processes can be sub-divided into:
Of these known processes for reducing the nitrate content, none has clearly dominating advantages. They all leave something to be desired in a technical and economic respect. Biological and ion-exchanger processes and reverse osmosis require the addition of chemicals, which is inconvenient, environmentally damaging and expensive. In addition, the biological process is very maintenance-intensive and requires considerable monitoring. In other processes, the nitrate removal is not specific enough.
Processes and apparatuses for carrying out electrodialysis, including removal of nitrates from solutions, are known. Furthermore, the treatment of nitrate-containing water by means of reverse osmosis and electrodialysis is known. The method of electrodialysis of aqueous solutions using selective membranes, the ions of relatively low charge being preferentially transported, is likewise known. Nevertheless, details on the membranes used, the current density, etc., are often lacking.
Certain homogeneous, thin, asymmetric or combination membranes have the property that either blocking by insoluble salts on the non-coated side in the concentrate chamber very easily occurs, and cannot be eliminated even by reversing the poles, or that the electrical resistance is too high in the case of homogeneous, thin membranes.
The prior art includes the following publications:
U.S. Pat. No. 3,510,417 and U.S. Pat. No. 3,510,418 describe the production of selective anion exchange membranes and cation exchange membranes which can be employed according to the invention.
Since the known processes for treatment of process water leave something to be desired, there is a great demand for the further development and refinement thereof.